Dolph Houben

15 Introduction and Outline of the Thesis 1 cells produce chemokines which recruit leukocytes and antigen-presenting cells (APCs) to the transplant site. The adaptive immune response is mediated by antigens presented by APCs (dendritic cells, macrophages), which undergo either direct or indirect T-cell allorecognition. At the same time, APCs from the allotransplant migrate from the allotransplant into the recipient and can therefore be involved in the adaptive immune response. The direct pathway is responsible for most acute rejection episodes, where the indirect pathway is responsible for chronic rejection. The direct pathway allorecognition is activated by MHC class II and I alloantigen’s recognized by CD4 and CD8 T cells directly presented by donor antigen-presenting cells (APCs, mostly dendritic cells). In the indirect pathway, the MHC antigens are first internalized by the APCs and thereafter presented as peptide fragments, and recognized by CD4 and CD8 T cells [72] . After activation of the T cells through binding of the antigen, they undergo a process of signal transduction, amplification, and production of transcription factors. These lead to proliferation and secretion of immunoglobulins and cytokines [73] . Reports of B cell responses or antibody- mediated rejection of VCAs are limited [60] . Drug therapy in VCA As discussed above, in clinical practice, suppression of VCA rejection is achieved through a comprehensive multi-level immunosuppressive drug therapy. Current immunosuppressive regimens are similar to those of solid organ transplantation, including a combination of calcineurin inhibitors (tacrolimus), antiproliferative agents (mycophenolate mofetil), and different dosing regimens of corticosteroids [74] .Thisblocks the formation, stimulation,proliferation, anddifferentiationof lymphocytes [75-77] . Prevention of musculoskeletal tissue transplantation rejection is even more problematic and requires 2-3 times greater immunosuppressive doses [78] . Inadequate drug treatment results in rejection and causes damage to the vascular endothelium, with subsequent increased vascular permeability and activation of leukocytes [79] . Thereafter, thrombosis of the allogeneic circulation occurs resulting in tissue death. Long-term drug therapy carries significant risks, including opportunistic infections, delayed wound healing, malignancy, metabolic diseases and end-organ toxicity [60, 74, 80, 81] . Thus, life- long IS presents an ethical dilemma in non-life-critical tissue transplantations. Tolerance induction and allotransplantation Induction of donor-specific tolerance is considered the ‘holy grail’ of organ transplantation, providing a method of maintaining long-term transplant viability without the use of drug therapy. The concept of engraftment of donor bone marrow cell in host tissue (mixed chimerism) to induce donor-specific allograft tolerance was first described in the 1950s [82, 83] . Tolerance induction by the introduction of mixed chimerism is believed to be of importance by many researchers [84-86] . This theory has therefore been tested in many experimental settings with various outcomes. Previous research form our laboratory used skin-grafting as a means for evaluation of immune competence and for demonstrating the absence of donor-specific tolerance after bone only VCA [87] . Additonally, donor-specific tolerance has rarely been achieved clinically and is therefore controversial [88] . Another proposed approach to induce tolerance is the use of regulatory T cells, thus suppressing the rejection response. This has led to drug-free transplant survival in three patients for a limited period of time [89, 90] . Thymic clonal deletion of responding immunocytes, use of costimulatory blockade [91, 92] , low-dose radiation, infusion of antilymphocyte serum combined